Tumblr founder David Karp, in a long-ish blog post, issues something between a mea culpa and a clarification of the site's byzantine porn rules (all emphasis added):

Last year, we added “Safe Mode" which lets you filter out NSFW content from tag and search pages. This is enabled by default for new users and can be toggled in yourDashboard Settings. As some of you have pointed out, disabling Safe Mode still wasn’t allowing search results from all blogs to appear. This has been fixed.

...

Earlier this year, in an effort to discourage some not-so-nice people from using Tumblr as free hosting for spammy commercial porn sites, we started delisting this tiny subset of blogs from search engines like Google. This was never intended to be an opt-in flag, but for some reason could be enabled after checking off NSFW → Adult in your blog settings. This was confusing and unnecessary, so we’ve dropped the extra option.

To summarize: Tumblr was trying to sneak porn out through the back door by making it harder to find, whether by tag pages (#dicks) or a simple Google search. That shouldn't be the case, at least as much—it's still vague, perhaps intentionally so. Karp calls these changes "fixes," as if they were bugs, and says "there haven’t been any recent changes to Tumblr’s treatment of NSFW content." That might be true. But Yahoo's treatment, and Madison Avenue's treatment, is what matters. Dog porn and rape fantasies don't jibe well with Lululemon GIFs.

If I'm not mistaken, NSFW content isn't solely comprised of porn. There are plenty of photographers out there that do nude photo shoots. There was fear amongst by tumblr friends that restrictions would shut down a lot of quality photography blogs.

Just clarification for anyone who thinks this is just people desperately trying to cling to their badly photoshopped pictures of Selena Gomez's head on a porn stars's body.